The present invention refers to improvements to liquid fuel burner ignition devices and to a control apparatus for said burners which incorporate said improvements.
The invention particularly relates to a burner control apparatus as described in Italian Patent Application Ser. No. 30885 A/75 of the Applicant. Essentially the apparatus comprises a motor actuated by a heat request signal generating means, a fan for feeding air to the burner, a pump for feeding liquid fuel to the burner, a valve means which inhibits the flow of liquid fuel from the pump to the burner in a "closed position" and permits such flow when in an "open position", a means for producing a spark used for igniting the liquid fuel which flows to the burner, which comprises a high frequency ignition transformer having a secondary winding which feeds the ignition electrodes for forming the spark, and a means for feeding the primary winding of the high frequency transformer with a low voltage of a high frequency. The means for feeding the primary winding of the transformer comprises an oscillating circuit which is fed with unidirectional current. The unidirectional current is in turn obtained by rectifying a low voltage and low frequency alternating current, generally having a line frequency, preferably drawn from the motor winding.
Summing up, therefore, a low voltage of a low frequency generally having a line frequency, is rectified to obtain a unidirectional low voltage current, and the rectified current is fed to an oscillating circuit which produces low voltage alternating current of a high frequency with which the primary winding of a transformer is fed; the secondary winding of the transformer furnishes a high voltage of a high frequency to the electrodes for producing the spark used for igniting the fuel.
In ignition devices of this type, in general, it is necessary to provide a flame detection means for determining the suitable operations according to whether the flame forms or does not form in the ignition stage or, after it has been formed, whether it disappears during the operation. If the flame forms, then such a means cause the burner to pass from its ignition mode to its regular stable operating mode, whereby the spark ceases to be formed and a permanent flow of fuel is effected. If the flame does not form, all this does not occur and, through the intervention of other devices, the burner is locked out after a certain preset safety period of time has passed. On the contrary, if the flame forms but goes out during the operation, then the detection means cause the ignition cycle to be repeated, whereby the aforesaid consequences are produced if the flame respectively forms or does not form.
In the cited previous patent of the Applicant, and actually in the invention to be described as well, the presence or the absence of the flame is detected by a photo-sensitive device, e.g. - a photo-cell or a photo-resistance. However, in the known device, and in particular in that of the cited previous patent, the detection of the photo-sensitive device is not fully satisfactory because only its response to a certain light intensity is exploited and not its response to lower light intensities, viz. only one degree of sensitivity is exploited, so that the device can not respond as desired to abnormal operating conditions.
An object of the present invention is an improvement in the flame surveying or detection means and related ignition device controls, which permits the exploitation of two different degrees of sensitivity or levels of a photo-sensitive or photo-revealing device, in different operating modes. More particularly, said different sensitivity levels are such that the sensitivity in the starting phase of the device is higher - i.e. the sensitivity level is lower - than in the operating mode. This behaviour may be defined as a "negative differential sensitivity", which is desirable and is adopted for compliance with regulations and for enabling greater safety. By the expression "photo-sensitive device" is meant herein a configuration of elements or circuit means which comprise a photo-sensitive device (a photo-cell or photo-resistance) which transmits to the other parts of the ignition device the suitable operative signals, as a function of the light intensity of the flame.
It is to be remembered that the operational cycle of the burner ignition device develops starting from an initial pre-purging stage, after which, if the conditions are normal, the ignition spark is caused to form and concurrently the consent is given to enable the flow of the fuel to the combustion chamber, so as to create the conditions for the flame to form. Normally, the ignition of the air-fuel mixture is immediate; if there are abnormal conditions, the ignition attempt may last for a certain time (safety time) which may be e.g. on the order of 5 seconds, whereafter if the flame has not formed, the burner goes into a "block" mode, as is commonly said--viz. a device intervenes which discontinues the electrical feed to the entire apparatus. To produce the electrical contact once again, it is then necessary to manually act on a deblocking push button, after waiting for a certain time which may be about 20 seconds, so that the ignition cycle starts once again from the beginning. If the flame appears before the lock out time is ended, the ignition transformer goes out, the valve which enables the passage of the fluid is controlled so as to be permanently excited, thereby giving a permanent enablement of the outflow of the fuel, and the burner runs regularly until a thermostat stop it. If during the operation, the flame goes out or its luminosity decreases below the photo-sensitivity level, the fuel flow to the combustion chamber ceases and the ignition cycle is repeated from the beginning.
However, additionally, if the photo-resistance sees light during purging, either because something is out of order or because an extraneous light is present in the combustion chamber or elsewhere due to any abnormal condition, the burner must remain in the pre-purging phase without the feeding of the fuel.
The present invention involves a difference of sensitivity of the photo-sensitive device comprising e.g.-a photo-cell or photo-resistance, such that it is more sensitive, as has been said, in the pre-purging mode than in the operating phase. Therefore, even a light intensity which is lower than that which is necessary to maintain the fuel inlet valve actuated and therefore necessary to keep the burner operating, suffices to stop the pre-purging thermal relay, which causes the pre-purging to cease after a preset period of time, and therefore interrupts the counting of the pre-purging time so that the burner remains in this phase until the extraneous light disappears.